This invention relates generally to digital radio systems and more specifically to a system transmitting, receiving, and demodulating TDMA QPSK modulation in a multipath fading environment such as an environment where the transceivers may be in motion and are served from fixed site transmitters and receivers such as in a radiotelephone system. This invention is related to instant assignee's U.S. patent application Ser. Nos. (128,975) "Rapid Reference Acquisition and Phase Error Compensation for Radio Transmission of Data", filed on behalf of Labedz et al. and (129,976) "Phase-Coherent TDMA Quadrature Receiver for Multipath Fading Channels", filed on behalf of Borth et al. on the same date as the present invention and containing related subject matter.
In a typical environment, a UHF or microwave radio channel exhibits a multipath structure in addition to Rayleigh fading. Thus, a radio transceiver for a mobile or portable TDMA system operating at high data rates must accommodate reception of multiple replicas of the transmitted signal, each with a random magnitude, phase, and time delay with respect to the transmitted signal. Without corrective measures, the data message can be obliterated by the multipath signals. As early as 1958, a receiver capable of accommodating these impairments was described for the use of either Differential Binary Phase-Shift Keying (DBPSK) or noncoherent Frequency-Shift Keying (FSK). It employed a channel sounding method to estimate the channel impulse response or channel profile, and a transversal equalizer having taps which were adjusted in response to the estimated channel profile.
By 1960 the multipath channel had been exhaustively studied and simulated, and optimum reception had been defined, but largely avoiding phase coherent techniques. Such demodulation techniques do not permit the use of higher spectral efficiency modulation methods which employ two-dimensional signaling techniques such as shaped Quadrature Phase-Shift Keying (QPSK) and its variations.
By 1983, TDMA (Time Division Multiple Access) receivers for digital telephony using Binary Phase-Shift Keying (BPSK) phase coherent detection had been described in U.S. Pat. No. 4,587,662. In 1985 this was extended to include QPSK, but the receiver was only described in general terms.
In 1986 an MSK receiver, with possible application to QPSK, was reported which could accommodate two rays of multipath and which used an adaptive equalizer employing both feedforward and feedback filtering. (See Krister Raith et al., "Multi-Path Equilization for Digital Cellular Radio Operating at 300 kbits/s", 36th IEEE Vehicular Conference, pp.268-272, May 1986). Although this adaptive equalizer apparently has never been thoroughly described in the literature, it is different than the multipath correlation employed in the present invention since it requires decisions to be made on the output in order to adjust the equalizer.
Adaptive equalization generally operating continuously on the data being received has been utilized in digital microwave receivers receiving continuous data streams. Such continuous receivers can equalize over a relatively long period of time. TDMA, due to its burst-like characteristics, demands rapid determination of the channel profile including the significant multipath contributions. Even though the data transceiver may be moving, the channel profile can be assumed to undergo negligible changes in a given TDMA timeslot if the timeslot is sufficiently short in duration.